A privacy issue has been corrected on the Steam Community Website that could expose details about users' games and activity, even when their profiles were set to private or friends-only. Ars Technica says they brought the problem to Valve's attention, and while it was never publically acknowledged, it is not longer possible to exploit it.

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Who gives a shit about their activity feed being viewable by the public anyway? It's games you own, games you've been playing and earning achievements for, etc. That is not a privacy issue. If you're posting sensitive information in comments on your screenshots or something, that's your own stupidity.